I highly doubt that you can run 550W through the PCI-E cable without any consequences.
Correct me if i'm wrong, i'm not really following this wire debate, but there are 2 PCI-E slots on each board, so each PCI-E cable should be able to provide around 300W.
I don't see a problem with that.
The discussion was about the general characteristics of 18AWG wire and not specifically for the Hashfast board.
Still 300W per one PCI-E cable with 3x 12V lines is pushing it to the limit according to my experience - the cable gets quite hot.
For example KNC specifically says that: You need to have 1 PCI-E cable for each asic card.
2x 6-pin PCIe cables are used to power my Bitfury M board. A few people have had their's go up in flames when using between 500-600w. The three 12V molex contacts on the 6-pin connector themselves are good for 300W or so all combined, so long as your PSU manufacturer doesn't cheap out on them and use a smaller gauge than spec (which seems to happen on occasion).
It shouldn't be a problem if we're all getting seasonic psus.
Most probably it would be OK for the long term with a good quality PSU, still I prefer to limit the current on a single 18AWG strand to 6A = 18A for a single cale.